An aircraft propulsion system includes a nacelle in which a powerplant is arranged in an essentially concentric layout connected to the rest of the aircraft by a strut.
As shown in FIG. 1, the nacelle has an air intake (10) at the front that allows the air flow to be channeled in the direction of powerplant (12).
The air intake (10) is comprised of a lip (14) whose surface in contact with the aerodynamic flow is prolonged in the interior of the nacelle by an internal conduit (16) and on the exterior of the nacelle by an external wall (18).
The air intake (10) is connected to powerplant (12) by a fastening device illustrated in detail in FIGS. 2, 3A, and 3B. This fastening device is comprised at the powerplant level of a first annular flange (20) attached to a second annular flange (22) of a panel delimiting conduit (16) or of in-between piece (24), called a strap, connected to the panel delimiting conduit (16), as shown in FIG. 2. The two flanges (20) and (22) are placed one against the other and kept there by fastening elements, for example bolts or rivets (26), that go through flanges (20) and (22) and extend parallel to the longitudinal axis of the nacelle.
In accordance with the mode of production illustrated in FIG. 3A, the bolts or rivets (26) are made up of a shank (28) whose diameter can be adjusted to that of the passage holes made in the annular flanges (20) and (22).
In accordance with a second mode of production, shown in FIG. 3B, the passage 5 holes made in annular flanges (20) and (22) can be slightly larger in diameter than that of the shank (28) of the bolts or rivets (26). This play, of around 1 mm, between the passage holes and the bolts or rivets, allows relative movement between the two attached pieces.
In both cases, the passage holes are cylindrical.
The fastening device and especially the bolts or rivets (26) are sized to offset any possible risk of incidents, such as, for example, the fracture of a fan blade.
In that case, the conduit of the powerplant can be deformed along all or part of its circumference. At the time of the deformation, the passage holes of the annular flange of the powerplant are no longer in line with those of the air intake. In this configuration, the bolts or rivets (26) are particularly subject to some relatively strong shear stresses, significantly greater than the stresses present under normal operations. Even if use of the second mode of production has allowed relative movement between the two joined pieces as the result of the play present around the bolts or rivets (26), this play is clearly less than the relative movement between the two joined pieces occurring in the case of an incident like the fracture of a blade. In the case of the use of the second mode of production with play, it can be noted that the shear stresses are at least equal to or greater than those present when the first mode of production is used.
In order to be able to resist such stresses, the fastening device consists of a given number of bolts or rivets (26) with a given diameter.
Taking into consideration the resistance of a bolt or rivet (26) in an installation that conforms to the modes of production illustrated in FIGS. 3A and 3B, this leads to planning for a great number of bolts or rivets (26) in the fastening device and/or bolts and rivets (26) with a large diameter which results in a greater embarked mass and consequently greater aircraft fuel consumption.
According to another scenario, the powerplant conduit deformations tend to spread in the direction of air intake conduit (16). As a result, during the design of air intake conduit (16), which is generally made up of composite material and includes a noise suppression system, one must take into consideration these possible deformations.
To limit the spread of the deformation of the powerplant annular flange (20) towards the air intake annular flange (22), it is possible to provide a filter at the level of bolts or rivets (26). For each fastening device, this filter is comprised of at least a deformable sheath (30) slipped on to the shank (28) of bolt or rivet (26). As seen in the example shown in FIG. 3A, the deformable sheath is inserted between the annular flange (20) connected to the powerplant and a nut (32) of the bolt or rivet (26). This deformable sheath (30) has an internal diameter adjusted to that of the shank (28) and is comprised of a central portion of relatively little thickness, in order to allow it to deform, especially by buckling. This lay-out allows an increase in the energy absorbed by the deformation of the fastening device. It also allows limiting the spread of the deformations along the direction of the axis.
However, in case of the fracture of a blade, the greatest deformations are oriented in a radial direction. However, the effects of the deformable sheath (30) in that direction are limited.